The present invention relates generally to ultrasonic diagnostic systems, and, more particularly, to an ultrasonic diagnostic system that is capable of electronically adjusting the apparent origin of ultrasound scan lines from a transducer.
Ultrasonic transducers and imaging systems have been available for quite some time and are particularly useful for non-invasive medical diagnostic imaging. Ultrasonic transducers are typically formed of either piezoelectric elements or of micro-machined ultrasonic transducer (MUT) elements. When used in transmit mode, the transducer elements are excited by an electrical pulse and in response, emit ultrasonic energy. When used in receive mode, acoustic energy impinging on the transducer elements is converted to a receive signal and delivered to processing circuitry associated with the transducer.
The transducer is typically connected to an ultrasound imaging system that includes processing electronics, one or more input devices and a suitable display on which the ultrasound image is viewed. The processing electronics typically include a transmit beamformer that is responsible for developing an appropriate transmit pulse for each transducer element, and a receive beamformer that is responsible for processing the receive signal received from each transducer element.
An ultrasonic transducer is typically combined with associated electronics in a housing. The assembly is typically referred to as an ultrasonic probe. Typically, ultrasonic probes are classified as either one-dimensional (1D) probes having a single element wide array of elements, or two-dimensional (2D) probes having a multiple element wide array. Furthermore, a probe referred to as a xe2x80x9cbi-planexe2x80x9d probe includes two orthogonally positioned 1D arrays that may or may not intersect. A relatively new 2D probe, referred to as a xe2x80x9cmatrix probexe2x80x9d includes transducer elements arranged in two dimensions where each element is individually controllable, resulting in an ultrasound probe the scan lines of which can be electronically steered in two dimensions. Each dimension of a matrix probe can be thought of as a stack of contiguous linear arrays.
A matrix probe can comprise either a xe2x80x9cfully sampledxe2x80x9d or a xe2x80x9csparsely sampledxe2x80x9d aperture. In a fully sampled aperture, every transducer element is individually addressable and controllable, and all elements are contiguous. In a sparsely sampled aperture, a subset of the physical set of transducer elements is addressed and controlled, or equivalently, there is a pattern of physical gaps between some elements such that they are not all contiguous. Sparsely sampled 2D arrays allow for fewer system connections (fewer channels) while still achieving distribution of the acoustic elements in two dimensions. However, a significant drawback of sparse 2D arrays is the loss of ability to control scan beam shape.
A 2D-matrix probe can be used to develop three-dimensional (3D) ultrasound images. FIG. 1 is a schematic diagram illustrating the manner in which an existing ultrasound probe interrogates a volume. Ultrasound data is typically acquired in frames, where each frame represents one or more sweeps of an ultrasound beam emanating from the face of the probe 100. The probe 100 includes a two-dimensional array of transducer elements; an exemplar one of which is illustrated using reference numeral 103. Such a sweep is typically developed by generating a large number of individual scan lines along one scan plane. An example of one scan plane, or xe2x80x9cslice,xe2x80x9d is illustrated using reference numeral 102 and the scan plane comprises individual scan lines 108-1 through 108-n. In this case, each slice is in the shape of a sector, and the xe2x80x9coriginxe2x80x9d 101 of each scan line is located at the center of the surface of the physical face of the probe 100.
The scan lines are typically steered in 2 dimensions during scan sweeps to create a set of rastered scan slices, exemplar ones of which are illustrated as slices 102, 104 and 106, where each slice interrogates a 2-dimensional xe2x80x9csector regionxe2x80x9d of the field of view. In effect, each slice 102, 104 and 106 represents a traditional two-dimensional sweep, with each sweep being displaced in elevation from the neighboring sweep. Those having ordinary skill in the art will recognize that trapezoidal or parallelogram shapes can be generated for each of the slices instead of sectors. Furthermore, a large number of such slices, slightly displaced in elevation, can be used to interrogate a volume.
Assembling the data from the sector slices produces a three-dimensional set of data referred to as a scan volume. Since all of the lines originate from the same point, the rendered 3D volume appears as a pyramid or cone, where the apex of the volume is the scan origin at the transducer probe face, which is located at the patient""s skin surface.
When conventional ultrasound imaging systems develop this volume scan, they typically generate multiple slices in at least two dimensions. These multiple slices generate ultrasound data for the volume occupied by the slices. To produce three-dimensional images, this volume of data is then processed by the ultrasound imaging system to create an image for display on a two-dimensional surface (such as the surface of the CRT type display) that has the appearance of being three-dimensional. Such processing is typically referred to as a rendering.
Unfortunately, the existing xe2x80x9cpyramidxe2x80x9d or xe2x80x9ccone-shapedxe2x80x9d sector scan format limits the field of view near the skin surface. This situation is illustrated in FIG. 2, which is a graphical representation of one of the ultrasound slices of FIG. 1. The slice 102 includes scan lines 108-1 through 108-n emanating from the origin 101 of the transducer probe 100. Typically, the transducer probe rests against the patient""s skin, thereby providing a limited field of view near the transducer probe. This xe2x80x9cnear fieldxe2x80x9d is illustrated in FIG. 2 as the areas denoted 210. These near field areas 210 are beyond the maximum steering angle (theta (xcex8)) 202 achievable using the scan lines 108-1 through 108-n. This can be a problem in cardiac scanning when the probe is near the apex of the heart, such as with an apical 4 chamber view. In such a view, it is often desirable to scan and display a wider field of view near the apex (origin 101) than is allowed by the scan format. A wide field of view near the probe face is also desired for certain abdominal and peripheral vascular imaging but, is not available when the sector slice format is used to generate the 3D image.
Further, the xe2x80x9cline densityxe2x80x9dxe2x80x94defined as the number of scan lines per angle step in a slicexe2x80x94is adversely constrained by the scan format. To achieve sufficient spatial sampling in the far field (away from the probe face), the line density is increased: more lines per angle step. This results in wasteful oversampling in the near field, where the lines crowd together near the origin.
Another drawback of conventional ultrasound scanning systems is when using contrast agents in ultrasound scanning. Several problems are posed by the restriction to scanning a set of sector slices. The problems share a common root cause, which is the uneven distribution of acoustic power in the scanned volume. For example, in the near field, because scan lines are denser, there is disproportionate destruction of contrast agent microbubbles compared to destruction in the far field. Contrast agent applications, which evaluate body functions by measuring the concentration of contrast agent in tissue, can produce distorted results due to the uneven pattern of microbubble destruction.
Further, the generation and measurement of harmonic resonance in tissue echoes, a technique now widely used to improve image quality, is undesirably constrained by the sector scan format. Harmonic resonance generation is dependent on many factors in the ultrasound scanner, including transmit frequency, output pulse power, as well as factors in the medium itself. Many of these factors are already constrained by hardware limitations, FDA power limits, and the variability of human tissue. Manipulation of the scan line spacing and direction can compensate for some of these factors. However, the sector format generally prevents such manipulation.
Therefore, it would be desirable to have an ultrasound imaging system capable of displaying three-dimensional data in both near field and far field, while overcoming the above mentioned shortcomings of the conventional sector scan format.
Embodiments of the invention include a system and method for individually varying the orientation of scan lines in at least two dimensions. In one embodiment, the invention includes a system for generating a three-dimensional ultrasound volume scan, comprising a two-dimensional transducer probe having elements arranged in a plurality of dimensions, and a system controller capable of generating a scan line apparently emanating from a location other than the geographic center of the transducer probe.